Lamp having controllable spectrum

ABSTRACT

The spectral energy characteristic of a discharge lamp is controlled by changing the density of the fill substance. The spectral characteristic can be shifted while substantially maintaining its shape by changing the density of the fill. A sulfur or selenium containing discharge lamp which is operated at a pressure of at least about 1 atmosphere contains a low ionization potential substance in the fill. Characteristics which are improved are one or more of spatial color uniformity, extinguishing characteristics, and bulb starting reliability. Particular substances which are added to the fill are alkalai metal containing substances, III B metal containing substances, and alkaline earth metal containing substances. When light is reflected back into the bulb, the light which is re-emitted is stronger in the higher wavelengths.

The present application is a continuation of U.S. application Ser. No.060,553 filed May 13, 1993, abandoned which in turn is acontinuation-in-part of U.S. application Ser. No. 867,551, filed Apr.13, 1992, abandoned U.S. application Ser. No. 875,769 filed Apr. 29,1992, abandoned and U.S. application Ser. No. 882,409, filed May 13,1992, abandoned each of which application is a continuation-in-part ofU.S. application Ser. No. 07/779,718, filed Oct. 23, 1991, abandonedwhich is a continuation-in-part of U.S. application Ser. No. 07/604,487,filed Oct. 25, 1990.

One aspect of the present invention relates to an improved visible lamp,and particularly to such a lamp which has a controllable spectraloutput.

As is well known, the color of the light which is provided by a lamp isdetermined by the spectral energy distribution of the emitted radiation.In general, visible light sources emit over the spectral range of350-750 nanometers.

It is desirable to be able to control the "tint" of a lamp whichnominally emits over the entire visible spectrum. For example, forcertain applications it may be desirable for the light to be tinted red,while for certain other applications, a green tint may be preferred. Inaddition to being able to provide different lamps having differenttints, it is also desirable to be able to vary the tint or spectralemphasis of the light which is emitted by a particular lamp duringoperation.

In the prior art, discharge lamps are typically provided with differentspectral emphases by employing fill additives. For example, a metalhalide lamp which is doped with thallium emphasizes the green part ofthe spectrum, whereas one which is doped with sodium would emphasize theyellow. One disadvantage of such lamps is that a different additive orcombination of additives must be used to make each differently tintedlamp, thus introducing manufacturing complexities. Additionally, due tothe fact that different fill substances have different agingcharacteristics, the spectra of lamps using additives are prone tochange over time.

Another approach to modifying the color output of a lamp is to useexternal filters. However, such devices inevitably reduce the efficacyof the overall lamp system. It is also known that incandescent lamps canbe made more red by reducing the operating temperature of the filament,but this also has the effect of reducing the lamp efficacy.

Additionally, the above schemes which are known to the prior art changethe color emphasis of the light output by changing the shape of theoverall spectral distribution, i.e., by emphasizing one portion of thespectrum but not others. However, it has been found that for certainapplications, it is advantageous to change the color emphasis whileretaining substantially the same shape for the overall spectraldistribution. For example, in red/green/blue (RGB) color reproductionsystems such as a liquid crystal display (LCD) high definitiontelevision, it is desirable to provide a lamp having a spectral energydistribution which can emphasize the blue or red without substantiallydistorting the shape of the overall distribution.

In U.S. application Ser. Nos. 779,718, filed Oct. 23, 1991, 604,487,filed Oct. 25, 1990, 882,410, filed May 13, 1992 and 08/071,027, filedJun. 3, 1993, now U.S. Pat. No. 5,404,076 all of which are incorporatedherein by reference, a new type of discharge lamp is disclosed whichuses a fill which contains a sulfur or selenium containing substance.The fill is present at a pressure of at least about 1 atmosphere, and isexcited at a relatively high power density. The lamp produces amolecular spectrum in the visible part of the spectrum at a relativelyhigh efficacy and has exhibited a long lifetime and a stable coloroutput over time.

While the lamp disclosed in the prior applications has many advantageousproperties, when not used in accordance with an aspect of the presentinvention, the spectral output or color temperature may vary around theperiphery of the bulb. It is of course desirable for many applications,for the spectral output to be uniform around the bulb surface, so thatall portions of the illuminating energy appear to be the same color.

It has also been found that the above-described spatial "colorseparation" effect may become more pronounced when the discharge lamp isoperated at low power levels. Furthermore, at such power levels the bulbmay extinguish or the discharge may retreat from the bulb walls.

In accordance with a first aspect of the present invention, a dischargelamp is provided which has a fill substance which emits primarilythroughout the visible part of the spectrum, and which has the propertyof having a visible spectral distribution which can be changed bycontrolling the density of the fill substance.

In accordance with a second aspect of the invention, a discharge lamp isprovided which has a fill substance which emits primarily throughout thevisible part of the spectrum, and which has the property of having avisible spectral characteristic which can be shifted in wavelengthwithout substantially changing the shape of the characteristic bycontrolling the density of the fill substance.

In accordance with a third aspect of the present invention, dischargelamps which have a sulfur or selenium based fill are provided withdifferent spectral emphases or tints by controlling the fill density ofa sulfur or selenium containing fill substance.

In accordance with a fourth aspect of the invention, discharge lampshaving visible spectral characteristics of substantially the same shape,but shifted in wavelength from each other, are provided by controllingthe fill density of a sulfur or selenium containing fill substance.

In accordance with a fifth aspect of the invention, the spectral outputof a discharge lamp having a sulfur or selenium containing fill iscontrolled in real time by controlling the fill density duringoperation. This may be accomplished by controlling the cooling which isapplied to the lamp bulb to condense more or less of the fill substanceout of the gaseous fill.

Additionally, in accordance with an aspect of the present invention, thebulb fill is constituted so as to obviate the above-described effects ofspatially varying spectral output or color temperature. Thus, the lampmay be configured so that it emits with a uniform color temperaturearound the bulb surface. Additionally, operation at lower power levelswithout extinguishing may be possible.

In accordance with a sixth aspect of the present invention, a substanceis added to the bulb fill which improves the spatial uniformity of thecolor temperature of the light which is emitted by the bulb.

In accordance with a seventh aspect of the invention, a substance isadded to the fill which improves the starting of the lamp.

In accordance with a eighth aspect of the invention, a substance isadded to the fill which allows the lamp to be effectively operated atlower power levels.

In accordance with a ninth aspect of the invention, a substance is addedto the fill which has a low ionization potential.

In accordance with a tenth aspect of the invention, a substance is addedto the fill which is an alkalai metal containing substance.

In accordance with a eleventh aspect on the invention, a substance isadded to the fill which is a III B metal containing substance.

In accordance with a twelfth aspect of the invention, a substance isadded to the fill which is an alkaline earth metal or a rare earthelement containing substance.

In accordance with a thirteenth aspect of the invention, mercury isadded to the fill.

In accordance with a fourteenth aspect of the invention, an improvedbulb is provided, which may be used in a discharge lamp.

It is known in the prior art to add the substance sodium, which is analkalai metal, to discharge lamps to provide spectral emphasis, and toarc lamps in particular to rectify a problem known as "arc constriction"wherein the discharge between the electrodes becomes undesirablyconstricted in certain regions.

It has been discovered in accordance with the present invention thatwhen a low ionization potential material is added to the fill of a highpressure lamp containing sulfur or selenium, the spatial uniformity ofthe color temperature of the light emitted by the bulb and/or thestarting characteristics of the lamp are improved.

It has further been discovered that when an alkalai metal containingsubstance is added to the fill of a high pressure lamp wherein theprimary light emitting fill component is a sulfur or selenium containingsubstance, the following advantages are provided:

a) the color temperature of the emitted light around the surface of thebulb becomes spatially more uniform, and this is in general true even atlower power densities;

b) the lamp starts more reliably; and

c) the lamp may be operated at lower power levels without extinguishing.

Additionally, it has been discovered that when a III B metal containingsubstance is added to the lamp fill of a high pressure sulfur orselenium containing lamp, the spatial uniformity of the colortemperature of the emitted light around the bulb is improved.

In accordance with a still further aspect of the present invention, ithas been found that when the light which is emitted by a sulfur orselenium based discharge lamp is reflected back into the bulb, theresultant light which is re-emitted is stronger in the higherwavelengths, i.e., the red portion of the spectrum. Therefore, by usinga reflector which is approximately spherical in shape, a lamp which isricher in red wavelengths may be provided. Additionally, by using awavelength selective reflector, such as a dichroic reflector, a lamphaving a spectral output which is more nearly equivalent to that of ablack body may be provided. Also, by the use of such wavelengthselective reflectors, lamps which are well suited to particularapplications may be provided.

The invention will be better understood by referring to the accompanyingdrawings wherein:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a microwave powered electrodeless lamp.

FIG. 2 shows a spectral energy distribution with a peak at 515nanometers.

FIG. 3 shows a spectral power distribution with a peak at 490nanometers.

FIG. 4 shows a graph of wavelength peak vs. fill density.

FIG. 5 shows a system wherein the fill density is controlled by varyingthe cooling of the lamp.

FIG. 6 shows a further embodiment of a high pressure sulfur or seleniumcontaining lamp.

FIG. 7 shows a further embodiment of a sulfur or selenium containinglamp.

FIG. 8 shows a exemplary spectrum emitted by the lamp of FIG. 6.

FIG. 9 is a graph of correlated color temperature vs. angular positionfor a lamp such as is shown in FIG. 6.

FIG. 10 shows a further embodiment of a lamp which may incorporate theinvention.

FIGS. 11 and 12 show further embodiments of sulfur or selenium baseddischarge lamps.

FIG. 13 shows a lamp with a spherical reflector in a spectrum measuringsetup.

FIG. 14 shows the spectrum obtained with the arrangement of FIG. 13 whenthe reflector is blackened.

FIG. 15 shows the spectrum obtained with the arrangement of FIG. 13 whenthe reflector is shiny.

FIG. 16 is a graph depicting the power ratios of shiny/black reflectorsby wavelength range.

FIG. 17 is a 1931 chromaticity diagram depicting coordinates of anuncorrected and corrected lamps respectively.

FIG. 18 shows a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a microwave powered electrodeless lamp into which thepresent invention may be incorporated. Referring to the figure, a pairof magnetrons 1, 1' generate microwave energy which propagates alongwaveguides 2, 2'. The waveguides lead to microwave cavity 5 which iscomprised of solid conductive wall, cup-shaped part 4, and metal meshcup-shaped part 6, which are joined at their respective ends 4A, 6A.Within the end of the waveguides 2,2', on the solid wall are locatedantenna slots 3,3' which serve to couple microwave energy from thewaveguide into the cavity where it causes an oscillating field to occur.

A discharge bulb 7 including a supporting stem 7A is located withincavity 5. The supporting stem is connected through a hole (not shown) insolid cup shaped part 4, to the shaft of a motor 8. The motor rotatesthe bulb 7 to improve the cooling of the bulb.

As mentioned above, the fill in bulb includes a sulfur or seleniumcontaining substance. It is further desirable to use an inert gas, suchas argon or xenon, which aids in starting the discharge. The lamp ofFIG. 1 may be characterized as a high pressure lamp. Thus, the fill inbulb 7 is present in amounts such that the fill pressure is at least 1atmosphere or above when excited to operating temperature, and ispreferably 2 to 20 atmospheres. Furthermore, the fill pressure is mainlycontrolled by the primary radiating component, which typically has asubstantially higher partial pressure than that of other constituentswhen the lamp is operational. The illumination provided by the lamp maybe augmented in various regions of the spectrum by including certainadditives in the fill.

As noted above, in addition to using sulfur and selenium in elementalform, compounds of these elements may be used. For example, InS, As₂ S₃,CS₂, SeO₂, SeCl₄, and HgSe, as well as other compounds of sulfur andselenium, may be used. The compounds which are used have a low vaporpressure at room temperature, i.e., they are in a solid or liquid state,and a high vapor pressure at operating temperature.

The term "a sulfur containing substance" as used herein, includes bothelemental sulfur and sulfur compounds, while the same is true for thecorresponding terms as applied to selenium. It should be appreciatedthat the primary radiating component of the fill may be comprised of acombination of a sulfur containing substance and a selenium containingsubstance, rather than only one of these substances. Additionally, theprimary radiating component may be comprised of a mixture of theelemental form and a compound(s) of a particular substance orsubstances.

The fill is excited at power densities in excess of 50 watts/cc andpreferably in excess of 100 watts/cc. As stated in application Ser. Nos.779,718 and 882,410 in the computation of power density, the volume,refers to the volume of the light emitting gas rather than to the volumeof the bulb.

Referring to FIG. 2, the spectral energy distribution of a lamp as shownin FIG. 1 is shown. The spectral power distribution shown in FIG. 2,covers the range from 350 to 750 nanometers, which is generally thevisible range. The spectrum has its peak at about 515 nanometers, andthe output appears to be white with a green tint. As is seen, thespectrum is continuous throughout the visible range. Analyses of thespectral energy distribution according to the 1931 CIE (CommissionInternational de l'Eclairage) determines a correlated color temperatureof 6000 degrees kelvin and x and y coordinates of 0.320 and 0.386respectively on the chromaticity diagram. The bulb which provided thespectrum which is shown in FIG. 2 was provided with a discharge fillconsisting of 2.5 milligrams per cubic centimeter of sulfur, and 60 torrof argon.

Referring to FIG. 3, the spectral power distribution of a second lampmade and operated according to the invention is shown. This lamp wasoperated under the same conditions as the lamp represented in FIG. 2,but the lamp represented in FIG. 3 was provided with sulfur of somewhatlesser density than the lamp represented in FIG. 2, that is 1.3milligrams per cubic centimeter. This bulb also had 60 torr of argon.The output appears white but in this case with a blue tint, and the peakof the spectral energy distribution occurs at about 490 nanometers. Thecorrelated color temperature is 8500 degrees kelvin, while the x and ycoordinates on the chromaticity diagram are 0.275 and 0.334respectively.

It should be noted that both of the spectra shown in FIGS. 2 and 3ascend from near zero smoothly from the 350 nanometer mark to theirrespective peaks, and descend more gradually to a low level at the 750nanometer mark. Aside from the slight jitter, the curves are smooth.This is in contrast to the ubiquitous variants of metal halide lampswhich exhibit strong line spectra. Additionally, it is significant tonote that the shape of the spectrum is substantially preserved betweenthe first and the second spectra. However, due to the spectral shift, itwill be seen that the amplitude of the spectrum shown in FIG. 2, whichpeaks at 490 nanometers is higher at the lower wavelengths and lower atthe higher wavelengths than that shown in FIG. 3, which peaks at 515nanometers.

Although the spectral energy distributions which are shown in FIGS. 2and 3 are significantly shifted from each other, they by no meansrepresent the extremes in what is available in the practice of theinvention. That is, lesser or greater amounts of fill can be used toachieve shifts in the spectrum toward shorter or longer wavelengthsrespectively. This is illustrated in FIG. 4, which is a graph of peakwavelength vs. sulfur density for the lamp which is shown in FIG. 1.

With regard to the choice between a sulfur containing substance, aselenium containing substance or combinations thereof, the following isto be noted. For a given fill density sulfur will provide a higher colortemperature and selenium a lower color temperature. Furthermore, a fillin which a combination of sulfur and selenium is used has the advantagethat higher total vapor pressures can be obtained from the two somewhatindependent partial pressures, and a further shift to the red can beobtained. Additionally, it has been determined that a fill comprising amixture of sulfur and selenium provides a spectrum having a shape asshown in FIGS. 2 or 3. It is contemplated that the relative shift of thespectrum attained with a bulb using both materials in the fill can becontrolled between the extremes of fills containing the only one of thematerials by selecting the ratio of sulfur and selenium in the fill.Increasing the sulfur density and decreasing the selenium density willraise the color temperature, and vice versa.

As mentioned above, such lamps may have particular applicability to ared/green/blue (RGB) color display system. The region of thechromaticity diagram extending from about 0.200 to about 0.490 on the xaxis, and about 0.200 to about 0.450 on the y axis is white light, whilevarious subregions will have a discernable tint. A source which fallswithin the white region is suitable for use in an RGB color system inwhich the light from the source is separated into the three primarycolor beams which are modulated imagewise and recombined to form a colorimage. It is however, desirable to be able to shift the spectrum whilepreserving its shape in order to provide an optimum spectrum for aparticular RGB system.

As mentioned above, various regions of the spectrum may be augmented byincluding certain additives in the fill. For example, one such fill,which is described in above-mentioned U.S. application Ser. No.07/604,487 in a bulb of 2.84 cm internal diameter utilizes 0.053 mg.moles/cc of sulfur, 0.008 mg. moles/cc of cadmium, and 0.003 mg. molesof cadmium iodide.

The lamp of the present invention has particular utility as a projectionsource, for example, for use in high definition television.

According to another embodiment of the invention, the spectral energydistribution of a particular lamp can be controlled during lampoperation. In this embodiment the effective fill density is changed byincreasing the cooling of the bulb, such as by increasing the pressureof the cooling air which is delivered to cooling jets 9A, 9B, 9C, 9D inFIG. 1 to the point that some of the fill in the bulb condenses on theinside of the envelope and ceases to participate in the discharge. Abulb may be modified to provide a special area or a side pipe may beprovided in which the fill material will be selectively caused tocondense. In this way the condensed fill will not interfere with thelight emission from the bulb. The special area may simply be a certainarea of an unmodified standard shaped bulb which is provided withaugmented cooling. For example in the lamp shown in figure one thecooling jet 9A which cools the lowest part of the bulb, whereat the stemis connected, may be operated at higher air pressure. In this way, fillcondensation will occur at that area of the bulb which is out of the wayof the emission directed at towards the lamps optics. Although not shownthe optics may comprise a reflector with its optical axis coincidentwith the cylindrical axis of the cavity.

A system for controlling bulb temperature is shown in FIG. 5, whichappears in U.S. Pat. No. 4,978,891, which is incorporated herein byreference.

Referring to FIG. 5, filter 30 is provided and is located so as toreceive light from the lamp. Filter 30, for example, may be a band passfilter which transmits light only in the blue region of the spectrum,and is followed by photodetector 32 which generates a comparison signal.

Function generator 34 is also provided, which is capable of generating apreselected function signal of desired, arbitrary shape. The outputs ofphotodetector 32 and function generator 34 are fed to comparator 36,which generates a difference signal. This difference signal is fed backto the cooling fluid supply system to control the amount of coolingfluid impinging on the bulb.

For example, in FIG. 5, an exemplary control for the cooling fluidsupply is a needle valve 40, the position of which is controlled bystepping motor 42. Alternatively, the input to pressurized air supply 20could be throttled or the supply could be vented, to control cooling.

Thus in accordance with this embodiment, whenever the output in the blueregion is different than that which has been programmed by functiongenerator 34, a difference signal results, which causes the cooling ofthe lamp bulb 8 to vary, until the difference signal is at or approacheszero.

Another method of changing the shifting spectrum would be to vary thepower while maintaining constant cooling. This would result in partialcondensation of the fill and would change the effective fill density inthe bulb resulting in a shift of the spectral energy distribution.

A combination of sulfur and selenium containing fill substances can beadvantageously used in the embodiments in which the spectrum is shiftedduring operation. Sulfur has a higher vapor pressure as well as asomewhat higher color temperature. As a discharge bulb containing bothsulfur and selenium is cooled to the point that a portion of the fillcondenses on the bulb 7 wall, the overall operational fill density willbe lowered leading to a higher color shift (i.e by way of a shift of thespectrum to the blue). A second compounding effect is that the seleniumwill condense out faster leaving an effective operating fill which hasmore sulfur, which by its nature gives a higher color temperature.

It should be noted that as used herein, the term "primary radiatingcomponent" means that fill component which provides a radiation outputwhich is at least as great as any other fill component.

It should be understood that the above embodiments have been illustratedin connection with specific lamps, but that other specific lampstructures may be used as well. For example, the shape of the microwavecavity as well as the microwave coupling schemes may be varied, and theinvention is also applicable lamps which are powered with radiofrequency (r.f.) energy, as well as to arc lamps.

While one advantage of the invention is that it provides a lamp which iscapable of operating without using mercury, the addition of a smallamount of mercury may help lamp starting. Additionally, for thoseapplications where the presence of mercury is not considered to be aproblem, it has been found that the addition of more substantial amountsof mercury increases efficiency significantly.

For example, in a bulb of 5 mm ID (6 mm OD), having a volume of 6.5×10⁻²cc, a mercury dose of about 5 mg was added to a sulfur dose of about0.33 mg, and 150 torr of argon. The pressure of the mercury vapor atoperating temperature was about 90 atmospheres.

At 570 watts microwave input power input, the addition of the mercuryresulted in an 11% improvement in efficacy, and had a significantlylower cooling requirement, which allows a less noisy pressurized coolingair source to be used. Such a lamp would be suitable for uses as aprojection lamp.

In accordance with a further embodiment, xenon is used as the inert gas,and is present at partial pressure during operation which is less thanbut comparable to the sulfur partial pressure within an order ofmagnitude, or a partial pressure which is greater than the sulfurpartial pressure. This arrangement results in increased efficacy.

For example, a bulb of 28 mm ID was filled with 24 mg of sulfur and 400torr of xenon at room temperature. An increase in efficacy of 6% wasrealized over the case where 60 torr of argon was used as the inert gas.

Referring to FIG. 6, a lamp embodiment which is used to illustrate afurther aspect of the present invention is depicted. Lamp 11 is anelectrodeless lamp which is powered by microwave energy. Bulb 12, whichcontains a high pressure fill, and is made of quartz or other suitablematerial, is supported in a microwave cavity, which is comprised ofconductive housing 13 and mesh 14. Magnetron 15 generates microwaveenergy, which is fed by waveguide 16, to coupling slot 17 of themicrowave cavity.

This excites the bulb fill to a plasma state, whereupon light is emittedby the fill, which is transmitted out of the cavity through mesh 14. Themesh is metallic, and is constructed so that it is substantially opaqueto microwave energy, while being substantially transparent to the lightwhich is emitted by bulb 12. The bulb is rotated by rotator 18, and thebulb envelope is cooled by gas which is fed in to plenum 19 and outthrough nozzles 19A.

A further embodiment of a lamp is shown in FIG. 7. This is an arc lamp60 which is comprised of quartz envelope 62 having electrodes 64 and 66,and containing fill 28. To excite the fill, an A.C. voltage is impressedacross the electrodes, whereupon an arc discharge occurs therebetween.

As in the case of the electrodeless lamp, the fill contains a sulfur orselenium containing substance which is present at a high pressure of atleast about 1 atmosphere and preferably in the range of about 2-20atmospheres. An electrical voltage is applied across the electrodes suchthat a suitably high power density exists. Additionally, the electrodes64 and 66 are made of or plated with a special material, to preventchemical reactions with the fill gas which may lead to electrodedeterioration. See U.S. application Nos. 604,487 and 071,027, now U.S.Pat. No. 5,404,076, which are incorporated by reference herein fordetails relating to operating parameters including power density andelectrode material.

The sulfur and selenium containing lamps described herein radiate amolecular spectrum in the visible region. A representative spectrum isdepicted in FIG. 8 and is seen to be smooth, with the sharp peaks whichare characteristic of atomic spectra being notably absent. The spectrumshown in FIG. 8 resulted when an electrodeless lamp such as is shown inFIG. 6 having a bulb of spherical shape of internal diameter 2.84 cm wasfilled with 0.062 mg-moles/cc of sulfur and 60 torr of argon, and wasexcited with microwave energy at a power density of about 280 watts/cc.

It has been observed that while light having the spectrum which is shownin FIG. 8 would be emitted from somewhere on the surface of the bulb,light having the identical spectrum would not in general be emitted fromevery point on the surface of the bulb. An accepted way of expressingspectral output is in terms of "color temperature" or "correlated colortemperature", and thus stated differently, it was found that the colortemperature or correlated temperature of the light varied as a functionof the angle from which the bulb is observed. This is depicted in FIG.9, which is a graph of correlated color temperature vs. observationangle. It is seen that as one progresses from the 0° line shown in FIG.6, through the 900 line, and then on to a displacement of 180°, thecolor of the emitted light changes. As mentioned above, for manyapplications, a bulb with a spatially uniform color temperature wouldprovide a better result. Additionally, the phenomenon of spatial "colorseparation" becomes more pronounced as the power level of the excitationenergy decreases, either by virtue of dimming the lamp, or operating itat a lower power. It was also found that it may not be possible to dimthe lamp as much as is desired, since the lamp may extinguish when a lowpower threshold is crossed.

It was further found that the above-described sulfur and/or seleniumcontaining lamps do not always start in a fast and reliable fashion. Inthe case of the lamp shown in FIG. 6, it is standard to use asupplemental light source to provide additional energy to initiatestarting. However, even with the use of such supplemental light source,starting may not always be reliable.

In accordance with an aspect of the present invention, a substance isadded to the bulb fill which has a low ionization potential. Such asubstance has electrons which are loosely bound, thereby making themeasy to dislodge. It has been found that when such a substance is addedto the bulb fill, one or more of the uniformity of the colortemperature, bulb extinguishing characteristics, and bulb startingcharacteristics, are improved.

One class of low ionization potential materials are the alkalai metals,and it has been found that when an alkalai metal containing substance isadded to the fill, the following advantages result:

a) the color temperature of the emitted light around the surface of thebulb becomes spatially more uniform, and this is in general true even atlower power densities;

b) the lamp starts more reliably; and

c) the lamp may be operated at lower power levels without extinguishing.

The alkalai metals may be used in either elemental or compound form, andone such substance which may be used is sodium. Other alkalai metals arelithium, potassium, rubidium, and cesium. By way of non-limitativeexamples, compounds in the form of halides or sulfides may be used, forexample NaI, Na₂ S or LiI. Adding a sodium containing substance to thebulb fill may also have the effect of providing spectral emphasis in thered part of the spectrum.

Another class of low ionization potential materials are the III Bmetals. It was found that when a III B metal containing substance wasadded to the fill, the color temperature of the emitted light becomemore uniform, and this is in general true even at lower power densities.Furthermore, it is possible to operate the lamp at lower power levelswithout extinguishing. The III B metals include indium, thallium,gallium, aluminum, and may be used in elemental or compound form, forexample combined as halides such as InI, TlI, TlBr, or combined assulfides such as InS, TI₂ Se, or TI₂ S.

A further grouping of low ionization materials are the alkaline earth orrare earth elements. Such substances cause the lamp to start morereliably. The alkaline earth metals are barium, beryllium, magnesium,calcium, strontium, and radium, and they may be used in elemental orcompound forms, for example combined as halides such as CaBr₂, BaI₂,SrI₂ and sulfides, such as CaS, BaS, BaSe. The rare earth elements areyttrium, scandium, and lanthanum through lutetium. The improved startingis due to the low work functions of the alkaline earth and rare earthelements.

It was further found that the addition of mercury to the bulb improvedstarting reliability.

The amount of the above-mentioned additives to be used to produceoptimum results for different applications may vary. For example, forthe lamp desired above which produced the spectrum of FIG. 8, Na may beadded in an amount of at least 0.001 mg/cc, In may be added in an amountof at least 0.01 mg/cc, and Ba may be added in an amount of at least0.005 mg/cc. Additives may produce spectral emphasis, so that theresultant spectrum may not be identical to that shown in FIG. 8. In thecase of mercury, at least 0.1 mg/cc should be used. It may also bepossible to use a combination of the additives disclosed herein togetherin the lamp fill. It is further noted that since some of theabove-mentioned additives improve lamp starting, it may be possible, incertain implementations, to eliminate the inert gas constituent of thefill.

The following Examples are illustrative:

EXAMPLE I

In a lamp as shown in FIG. 6, a bulb which is 2.84 cm in interiordiameter was filled with 24 mg of S (2 mg/cc) and 60 torr of Ar,, andoperated at a suitably high power density. A figure of merit identifiedas "uniformity", is defined as the ratio of the minimum to maximumintensity of the light outputted by the bulb considering all angularpositions except where the bulb is obstructed, e.g., by a narrow screenring. The reason that the "uniformity" is a figure of meritrepresentative of the color separation effect is that for lamps of thistype, regions of lower color temperature also have lower output. Theuniformity for the lamp was found to be 0.81.

EXAMPLE II

A lamp as described and operated in connection with Example I was filledwith 24 mg of S (2 mg/cc), 60 torr of Ar., and 0.2 mg of NaI (0.017mg/cc) which contained 0.031 mg (0.0026 mg/cc) of Na. The "uniformity"was 0.97, and a uniform color temperature around the angular extent ofthe bulb could be visually observed.

EXAMPLE III

A lamp as described and operated in connection with Example I was filledwith 24 mg of S (2 mg/cc), 60 torr of Ar., and 0.3 mg of InI (0.025mg/cc) containing 0.143 mg (0.012 mg/cc) of In. The "uniformity" was0.91.

EXAMPLE IV

A lamp as described and operated in connection with Example I was filledwith 24 mg of S (2 mg/cc), 60 torr of argon, and 1 mg (0.083 mg/cc) ofBaS containing 0.81 mg (0.068 mg/cc) of Ba or instead of the BaS, 7 mgof Hg. An improvement in the starting reliability of the lamp wasobserved.

It should be noted that in term "power density", the volume (cc) refersto the volume of the light emitting gas rather than to the volume of thebulb. Further, it should be noted that the term "operating temperature"as used herein is the temperature which is attained by the bulb duringoperation.

It is significant that when fill additives as discussed herein are usedit may be possible to operate the lamp at significantly lower powerdensities than indicated above without causing objectionable colorseparation or bulb extinguishment.

A further embodiment of the invention is shown in FIG. 10, which is anillustrative example of a lamp which is excited with electromagneticenergy in the radio frequency range. In this regard, the term"electromagnetic energy" as used herein refers to both microwave andr.f. modes.

Referring to FIG. 10, r.f. source 70 generates r.f. power which is fedto induction coil 72. Bulb 74, which houses a sulfur or seleniumcontaining fill as described above also includes additives which mayinclude an alkalai halide containing substance or a III B metalcontaining substance as discussed above. In the operation of the lamp,r.f. energy from the induction coil 42 is coupled to the bulb fill,thereby exciting it to produce a spectrum in the visible range aspreviously described. The additives mentioned above permit operation atlower power densities, which in general is a significant advantage, andmay be a particular advantage in the use of r.f. lamps. The inventionmay be applied to the different types of r.f. lamps, which include byway of non-limitative example, inductively and capacitively coupledlamps.

As is well known to those skilled in the art, the particular form of theelectrodeless lamp heretofore described is exemplary only, and otherspecific shapes and types of cavities, for example, substantially allmesh type, as well as different types of coupling modes using one ormore power sources, and one or more waveguides or other coupling modesmay be used.

For example, FIG. 11 illustrates a lamp wherein coupling is effected ina coaxial mode. Microwave power is provided to inner and outerconductors 82 and 84 for coupling to bulb 86. A conductive mesh 87 isconnected to the outer conductor. A tuning element 88 may be provided tohelp in starting the lamp.

FIG. 12 depicts a further embodiment which is powered by r.f. ormicrowave power. Power from high frequency power source 104 is coupledto inner conductor 107 and outer conductor 106, which is a conductivemesh, the bulb 101 is supported between inner conductive member 107A andinner conductive member 107B. The embodiment shown in FIG. 12 may beconsidered to be a form of capacitive coupling.

As discussed above, the sulfur and selenium containing lamps describedherein radiate a molecular spectrum in the visible region. Arepresentative spectrum is smooth, with the sharp peaks which arecharacteristic of atomic spectra being notably absent.

FIG. 13 depicts an embodiment of the invention wherein electrodelesslamp 110 is shown which has a fill wherein a sulfur containing substanceor a selenium containing substance is the primary radiating component,as described above. Bulb 112 is secured in approximately sphericalreflector 114 by bulb stem 115. The bulb stem may be arranged forrotation while streams of cooling fluid are directed at the bulb, toeffect cooling (not shown). A mesh 116 contains microwave energy whileallowing the emitted light to escape. Microwave energy is fed to thecavity via waveguide 118, and is coupled thereto through slot 119.

In accordance with an aspect of the invention, reflector 114 isapproximately spherical in shape. This causes the light to be reflectedby the reflector back into the bulb. The resultant light which isre-emitted from the bulb is stronger in higher wavelengths than in thecase where light is not reflected back into the bulb.

This was demonstrated by the experimental setup depicted in FIG. 13,wherein the light emitted by the lamp is fed through baffles 120 and121, which have co-linear openings 122 and 124 respectively disposedtherein. A diffuser 125 backed by monochromator 126 is located in linewith the baffle openings so as to receive the light coming therethrough.

Spectral measurements were taken for the case where the inside ofreflector 114 is blackened, so as to be non-reflective, and the casewhere the reflector is shiny. The bulb was 2 cm in inner diameter andwas filled with 2 mg/cc of elemental sulfur and 60 torr of argon, andwas excited at a power density of about 325 watts/cc.

The resultant spectrum for the case where the reflector is blackened isshown in FIG. 14, while the spectrum for the case where the reflector isshiny is shown in FIG. 15. The data is expressed in irradiance (powerper wavelength interval per square centimeter). As can be seen, not onlydoes the lamp with shiny reflector produce a more intense output, butthere is a greater concentration of higher wavelengths. This is shownmore exactly in FIG. 16, which is a bar graph of the output power ratioof the lamp for shiny/black reflector. It is seen that the power ratiois about a factor of 2 in the yellow region of the spectrum, and afactor of about 3 in the red region. Thus, a lamp which is rich in suchwavelengths may be produced by following the teachings of the invention.

Additionally, it was found that the output from the lamp with the shinyreflector was more uniform than that of the lamp with the blackreflector, and while the bulb in the shiny reflector ran hotter than thebulb in the black reflector, it did not run much hotter, so thatadditional light power was obtained with only a modest increase incooling requirement.

In accordance with a further aspect of the invention, selectivewavelengths may be reflected back into the bulb to cause the lamp toemit a spectrum which is more equivalent to the spectrum radiated by ablack body. For example, this may be effected with the use of dichroicreflectors in either discrete form, or disposed directly on the bulb inthe form of a coating.

For example, if a dichroic reflector in the form of an optically thinfilter which reflects only in the green region is disposed on the bulb,the output in the green region of the spectrum may be substantially cut,for example by a factor of 2. At the same time, the output in the redregion of the spectrum increases.

Thus, a lamp having a spectrum produced by a sulfur fill modified asdescribed above will emit more nearly like a black body. This is shownin FIG. 17, which is a 1931 chromaticity diagram. The position of theunfiltered output of such a lamp is shown at point D, where x=0.320 andy=0.371, while the position of the filtered output using a dichroicreflector which reflects only in the green region is shown at point E,which is at the black body line, where x=0.341 and y=0.346.

FIG. 18 shows an embodiment of the invention using a dichroicreflector/filter such as described above. In this embodiment, thedichroic reflector 130 is disposed on spherical bulb 92 which is locatedin reflector 134 which is closed by mesh 136. As known to those skilledin the art, such a dichroic reflector may be comprised of alternatinglayers of materials having different indices of refraction. For example,for reflecting in the green or green-yellow part of the spectrum around540 nm with about a 50 nm bandwidth, a dichroic reflector comprised of 5sets of alternating layers of zirconium oxide and silicon dioxide usinglayers 67.5 and 89 nm thick, respectively, may be used. As is known, thethickness and number of layers may be varied to change the spectral bandwhich is reflected.

In accordance with a further aspect of the invention, the spectraloutput of the lamp may be tailored for particular applications byreflecting back selected wavelengths into the bulb. For example, a lampin which green radiation is reflected back may be used for horticulturalapplications such as for plant growth as in greenhouses. Thus, thespectrum of the sulfur-based lamp is inherently high in greenwavelengths, and these are attenuated by the dichroic filter, while thered wavelengths, which are useful in inducing plant growth areincreased. Wavelengths other than green may be reflected back to producedifferent resultant spectral outputs.

It should be understood that while the invention has been illustrated inconnection with specific embodiments, variations will occur to thoseskilled in the art, and the scope of the invention is to be limited onlyby the claims which are appended hereto and equivalents.

We claim:
 1. A visible discharge lamp which provides a spectral powerdistribution having a peak at one of many possible peak wavelengthscomprising,an envelope which contains a fill having at least one visiblelight emitting component, wherein the visible light emitting componentwhich emits more visible light than any other such component which maybe present is a substance selected from the group consisting of sulfurand selenium, wherein said sulfur or selenium emits principally visiblelight, excitation power generating means, and means for coupling powergenerated by said excitation power generating means to said envelopefill, said lamp during excitation producing an output spectral powerdistribution in a continuous band having a peak at a certain wavelengthwhich corresponds to a first particular density of said sulfur orselenium, wherein other possible peak wavelengths correspond to otherparticular fill densities, and said lamp fill is provided with saidsulfur or selenium at said first particular density during excitation soas to result in said spectral distribution which has a peak at saidcertain wavelengths.
 2. The lamp of claim 1 wherein said lamp iselectrodeless.
 3. The lamp of claim 2 wherein said substance is sulfur.4. The lamp of claim 2 wherein said substance is selenium.
 5. The lampof claim 2 wherein the visible light emitted by the sulfur or seleniumis molecular radiation.
 6. A visible discharge lamp for producing adesired spectral output comprising,an envelope which contains a fillwhich includes at least one substance during excitation which isselected from the group consisting of sulfur and selenium, wherein saidsubstance emits principally visible light in a continuous band,excitation power generating means, and means for coupling powergenerated by said excitation power generating means to said envelopefill, said substance during excitation producing an output spectralpower distribution having a peak at a certain wavelength whichcorresponds to a particular density of said substance, wherein otherpossible peak wavelengths correspond to other particular densities ofsaid substance, and said lamp fill is provided with said substance atsaid particular density so as to result in said desired spectral output.7. The lamp of claim 5 wherein the visible light emitted by saidsubstance is molecular radiation.